The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale formations. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit interspersed with layers containing an organic polymer called "kerogen", which upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the carbonaceous liquid product is called "shale oil". A number of methods have been developed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement of disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous carbonaceous materials from a subterranean formation containing oil shale by mining a chamber in the formation, and explosively fragmenting and expanding formation around the chamber to form a cavity containing a stationary, fragmented, permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort.
Once the retort is thus formed in situ, the fragmented mass of formation particles is ignited at the top of the retort to establish a combustion zone in the retort. A combustion supporting processing gas comprising oxygen, such as air, is introduced into the combustion zone to sustain combustion of oil shale in the combustion zone and to advance the combustion zone through the retort. In the combustion zone oxygen in the combustion supporting processing gas is depleted by reaction with hot carbonaceous materials to produce heat, combusted oil shale, and combustion gas. By the continued introduction of the combustion supporting gas into the combustion zone, the combustion zone is advanced through the retort. An effluent gas from the combustion zone, comprising combustion gas and the portion of the combustion supporting gas that does not take part in the combustion process, passes through the retort on the advancing side of the combustion zone to produce retorted oil shale and to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, to gaseous and liquid hydrocarbon products.
As used herein, the term "retorted oil shale" refers to oil shale heated to a sufficient temperature to decompose kerogen in an environment substantially free of free oxygen so as to leave a solid carbonaceous residue. The term "combusted oil shale" refers to oil shale of reduced carbon content due to oxidation by a gas containing free oxygen. An individual particle containing oil shale can have a core of retorted oil shale and an outer "shell" of combusted oil shale. Such can occur when oxygen has diffused only part way through the particle during the time it is at an elevated temperature and in contact with an oxygen supplying gas.
As used herein, the term "processing gas" is used to indicate gas which serves to advance a processing zone such as a combustion zone, a retorting zone, or both a retorting zone and combustion zone, through an in situ oil shale retort. The term "processing gas" includes, but is not limited to, an oxygen supplying gas introduced into a retort for advancing a combustion zone and retorting zone through a retort, and a hot retorting gas which can be introduced into a retort or generated in a combustion zone in a retort for advancing a retorting zone through a retort.
The liquid hydrocarbon products and gaseous hydrocarbon products are cooled by the cooler oil shale particles in the retort on the advancing side of the retorting zone. The liquid products are collected at the bottom of the retort and withdrawn to the surface. An off gas containing combustion gas generated in the combustion zone, product gas produced in the retorting zone, and the portion of the combustion supporting gas that does not take part in the combustion process is also collected at the bottom of the retort and withdrawn to the surface.
The overall efficiency of the retorting operation is affected by the effectiveness of the explosive expansion of the formation to form a fragmented permeable mass. If the formation is not sufficiently fragmented, the total surface area of the particles in the fragmented mass is reduced and thus the rate at which the core portion of larger particles of the fragmented mass is heated to produce kerogen decomposition is reduced. In addition, unsatisfactory fragmentation can adversely affect resistance to gas flow through the fragmented mass. For economical retorting, it is desirable that gas passing through the retort have a unit pressure drop of less than about 0.01 or 0.02 psi. As used herein, the term "unit pressure drop" refers to the pressure drop or pressure gradient of gas passing through a fragmented permeable mass of formation particles containing oil shale at 1 standard cubic foot per minute per square foot of cross sectional area of the fragmented permeable mass per foot of advancement of the gas through the fragmented mass. Thus, for example, a retort with a length of 200 feet and unit pressure drop of 0.01 psi has a total pressure drop of 2 pounds per square inch when the flow rate of gas is one SCFM per square foot of cross sectional area of the fragmented mass. Above a unit pressure drop of about 0.02 psi, an undesirable amount of power is required to drive the gas blowers causing retorting gas to flow through the retort, particularly when long retorts such as retorts of 1,000 feet in height are being retorted.
After an in situ oil shale retort is formed, tests are made to determine the pressure drop through the fragmented permeable mass in the retort. If inadequate flow rate at a selected pressure gradient is obtained, one could either operate the retort as reduced efficiency, abandon the retort, or attempt to further fragment the mass of particles into smaller sizes to clear the blockage of gas flow.